Dispensing apparatus for a fluid

ABSTRACT

A dispensing apparatus is proposed for a fluid having a supply tank ( 2 ) for the fluid which has an outlet ( 4 ) which can be connected to a pressure line ( 5 ) for the fluid (F), and having a rotary pump ( 3 ) which has a rotor ( 31 ) for conveying the fluid (F) into the pressure line, with the rotor ( 31 ) for the mixing of the fluid being arranged directly in the outlet ( 4 ) of the supply tank ( 2 ).

RELATED APPLICATIONS

This application is a divisional application of copending application Ser. No. 10/309,941 filed Dec. 3, 2002, now U.S. Pat. No. ______.

BACKGROUND OF THE INVENTION

The invention relates to a dispensing apparatus for a fluid and to the use of such a dispensing apparatus.

There is a need in a number of industrial processes, for example in the manufacture of semi-conductors and chips, to dispense fluids in a controlled manner via nozzles or similar apparatuses. Chemical-mechanical polishing processes (CMP) such as are used in the semi-conductor industry can be named as examples here. In such processes, a suspension usually known as a slurry and typically made of very fine solid particles and a liquid is applied to a rotating wafer and there serves for the polishing or lapping of the very fine semi-conductor structures. Another example is the application of photo-resist to the wafer.

A dispensing apparatus suitable for this and known from the prior art is illustrated in FIG. 1. The dispensing apparatus 1′ includes a supply tank 2′ which is filled with the fluid, for example slurry. The supply tank 2′ has an outlet 4′ to which a pressure line 5′ is connected which extends via a recirculation pump R′ up to an inlet 6′ at the supply tank 2′. Downstream of the recirculation pump R′, a plurality of discharge points 7′ are provided in the pressure line 5′ which lead to nozzles or other apparatuses—usually known as tools—with which the fluid is applied to the wafers. Each discharge point 7′ is provided with a valve 8′ in order to open the flow connection to the respective apparatus. If all discharge points 7′ are closed, the recirculation pump R′ effects only a circulation of the fluid.

The desired pressure at which the fluid is transported to the tools through the pressure line 5′ and the open discharge points 7′ can be generated by applying pressure to the fluid in the supply tank 2′. For this purpose, an inlet 10′ is provided at the supply tank 2′ through which a pressure medium can be introduced into the supply tank via a pressure control valve 11′, as is symbolically represented by the arrow G. Usually a gas, e.g. nitrogen, is used as the pressure medium with which an overpressure of, for example, 0.5 bar is maintained in the supply tank 2′.

Such an apparatus, however, has disadvantages. To generate the overpressure in the supply tank 2′, this must be designed impermeable to gas, which requires quite an apparatus effort. Moreover, it is not easily possible to fill new fluid into the supply tank 2′ when the level becomes too low. A change in the pressure in the supply tank 2′ and thus a change in the forwarding pressure is also complicated and time consuming. It is furthermore possible for the pressure medium (gas) to penetrate the fluid or enter into a solution in the fluid, which can result in unwanted changes in the composition of the fluid. A further problem can occur in suspensions such as slurries or in fluids which tend to separation or clumping, because the circulation caused by the recirculation pump R′ is as a rule too low to ensure a fluid movement in the supply tank 2′ which is sufficient for a constant mixing. Additional measures are therefore frequently necessary to permanently ensure a sufficient movement or mixing of the fluid in the supply tank 2′.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a dispensing apparatus for a fluid which does not have the disadvantages discussed above. The dispensing apparatus should allow a sufficient mixing of the fluid and its availability in a pressure line in a simple manner.

In accordance with the invention, a dispensing apparatus for a fluid is therefore proposed having a supply tank for the fluid which has an outlet which can be connected to a pressure line for the fluid and having a rotary pump which has a rotor to convey the fluid into the pressure line, with the rotor being arranged directly in the outlet of the supply tank for the mixing of the fluid.

The rotary pump thus satisfies two functions: On the one hand, it conveys the fluid into the pressure line (pump function) and, on the other hand, the arrangement of the rotor directly in the outlet of the supply tank ensures a good and constant mixing of the fluid in the supply tank (stirring function). A precipitation or deposition of particles in suspensions, a clumping or a phase separation in the fluid can thus be effectively prevented.

It is advantageous for the best possible mixing for the rotary pump to have an inlet whose opening amounts to at least 30 percent, in particular at least 50 percent, of the diameter of the rotor.

A control unit for the rotary pump is preferably provided which sets the forwarding pressure of the rotary pump via the speed of the rotor. If the rotary pump is operated in an operating range with a low efficiency, there is a clear relationship between the speed of the rotor and the pressure at the outlet of the pump for a given fluid. This has the highly desirable advantage that the pressure at which the fluid is made available can be set or adjusted easily and in a very short time. A complex application of pressure to the fluid in the supply tank is thus no longer necessary.

With respect to a design which is as simple as possible in an apparatus aspect, it is advantageous for the rotor to be provided in a rotor housing which forms part of the wall of the supply tank.

The stirring function of the rotor can be positively influenced when the rotor is designed and arranged such that it projects at least partly into the supply tank.

The rotor preferably includes a plurality of vanes which extend into the supply tank. This can in particular be realized in that the vanes are oversized, that is, much larger, in comparison with known rotary pumps. The vanes thus also serve, in addition to generating pressure, as stirring elements which keep the fluid in the dispensing tank in motion.

It is also advantageous for the rotary pump to have a stator for driving the rotor, wherein the rotor is mounted magnetically in a contact-free manner with respect to the stator. Due to this measure, no seals are necessary at shaft bearings and the risk of damage to such seals, for example by abrasive particles, is avoided.

The rotary pump is particularly preferably designed as a bearing-free motor and the rotor as an integral rotor, because this represents a very compact and space-saving design.

In order to further improve the mixing and/or homogenization of the fluid in the supply tank, it can be advantageous to provide guide elements in the supply tank.

In a preferred use, the dispensing apparatus in accordance with the invention serves for the dispensing of suspensions, in particular of slurry, especially in a CMP process, or for the dispensing of photo-resist.

Further preferred applications of the dispensing apparatus in accordance with the invention are the determining of the viscosity of a liquid and the checking of properties of a fluid, in particular the checking of the mixing ratio in a fluid which is composed of a plurality of components.

The invention will be explained in more detail in the following with reference to embodiments and to the drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a known dispensing apparatus (prior art);

FIG. 2 is a schematic representation of an embodiment of a dispensing apparatus in accordance with the invention;

FIG. 3 is a variant for a dispensing tank; and

FIG. 4 is a schematic representation of a further embodiment of a dispensing apparatus in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a dispensing apparatus 1′, which represents prior art and was already explained at the start.

FIG. 2 shows in a schematic representation an embodiment of a dispensing apparatus in accordance with the invention which is designated as a whole with the reference numeral 1. The dispensing apparatus 1 includes a supply tank 2 for a fluid F which has an outlet 4. A rotary pump 3 having a rotor 31 is provided in the outlet 4 and is designed as a centrifugal pump here. The outlet 41 of the rotary pump 3 is connected to a pressure line 5 which extends from this outlet 41 of the rotary pump 3 via a pressure-reducing valve 9 up to an inlet 6 of the dispensing tank 2. The pressure line 5 has at least one—here for example three—discharge point 7 upstream of the pressure line 5 of which each is connected via a line 71 to an apparatus T for dispensing the fluid F, for example to a nozzle, or to a tool. A valve 8 is provided in each line 71 with which the flow connection between the removal point 7 and the tool T can be separately opened or closed.

The outlet 4 is arranged at the base of the dispensing tank 2. The opening of the outlet 4—by which its diameter is meant—is identical to the opening B of the inlet 30 of the rotary pump 3. This opening B is larger than half the diameter of the rotor 31.

In the following, reference will be made by way of example to an application particularly important for practice, namely that the dispensing apparatus 1 in accordance with the invention is used in a CMP process (CMP: chemical-mechanical polishing) in the semi-conductor industry. In these processes, a suspension known as a slurry of fine solid particles in a liquid is applied to a rotating wafer and there serves for the lapping or polishing of the very fine semi-conductor structures. The fluid F in this example is the suspension known as a slurry. The apparatuses or tools T each include a nozzle or another means with which the fluid F can be applied to the wafer.

By rotary pumps, which are also known as centrifugal pumps, there are meant all those pump apparatuses which have a rotor 31 or an impeller by whose rotation an impulse amount is carried out on the fluid to be conveyed. The term rotary pump includes in particular centrifugal pumps, axial pumps and side-passage pumps. The inlet and the outlet are typically in constant flow connection in a rotary pump. There are therefore, for example, no valves provided between the pump inlet and outlet 41.

In accordance with the invention, the rotor 31 for mixing the fluid F is arranged directly in the outlet of the supply tank 2. In this embodiment, the rotor 31 for mixing the fluid F projects at least partly into the supply tank 2. The rotary pump 3 thus not only serves for the pumping of the fluid F, but also as an agitator which mixes the fluid F in the supply tank 2. For this purpose, the rotor 31 has a plurality of vanes 311 which are designed much larger than with known rotary pumps of comparable dimensioning. As FIG. 2 and FIG. 3 also show, the vanes 311 extend into the supply tank 2 and here (when the rotor 3 is rotating) provide for a circulation of the fluid as is indicated by the arrows Z.

The representation of the vanes 311 in FIG. 2 and FIG. 3 is naturally only to be understood as an example. The vanes can have still further necks or larger surfaces or other suitable means in order to positively influence the stirring function.

The rotor 31 is arranged in a rotor housing 312 which forms a part of the wall of the supply tank 2. The rotor housing 312 can be an integral component of the supply tank 2 or be secured to this as a separate part.

The rotary pump 3 further includes a stator 32 having a stator winding 322 in order to electrically drive the rotor 31. Furthermore, a control unit 12 is provided which controls and regulates the rotary pump 3. The stator 32 surrounds the rotor housing 312. The stator 32 is preferably designed as the stator of a so-called temple motor. This means (see FIG. 2 and FIG. 3) that the stator 32 has a plurality of stator teeth connected by a return yoke, with each stator tooth being formed in an L shape with a shorter and a longer limb. The longer limb in each case extends parallel to the axis of rotation of the rotor and the shorter limb extends radially inwardly towards the axis of rotation. The longer limbs carry the stator winding 322.

In particular for such fluids which include solid particles or which are of high purity, the rotary pump 3 preferably has a completely magnetically mounted rotor 31; that is, the rotor 31 is magnetically mounted in a contact-free manner with respect to the stator 32. The absence of mechanical bearings for the rotor 31 has a plurality of advantages. For instance, the problem is avoided that abrasive particles can damage mechanical bearings. There is furthermore no risk of contamination of the fluid by lubricants or bearing abrasion. Sealing problems are also avoided.

It is particularly simple from an apparatus and energy standpoint for the rotor 31 to be permanently magnetic. For this purpose, the rotor 31 includes a permanent magnet, for example a permanently magnetic ring 313. This ring 313 is arranged around a central bore 314 which extends along the desired axis of rotation of the rotor 31 and through the rotor. The magnetization of the ring 313 is indicated by the arrows (without reference numerals) at its interior.

A particularly preferred rotary pump is disclosed, for example, in EP-A-0 819 330 or U.S. Pat. No. 6,100,618. This rotary pump has a so-called integral rotor and is designed as a bearing-free motor. The term integral motor is to be understood to mean that the pump rotor and the rotor of the motor driving the pump are identical. The rotor 31 operates both as a rotor of the motor drive and as a rotor of the pump. The term bearing-free motor is to be understood to mean that the rotor is mounted in a completely magnetic manner, with no separate magnetic bearings being provided. The stator 32 is both the stator of the electrical drive and the stator of the magnetic bearing. For this purpose, the stator winding 322 includes a drive winding of the polar pair number p±1. It is thus possible both to drive the rotor 31 and to mount it magnetically in the stator in a completely contact-free manner. Reference is made to the documents already cited with respect to further details of such a rotary pump.

During operation, the stator winding 322 controlled by the control unit 12 generates a drive rotary field which applies a torque onto the rotor 31 and sets this into motion. Furthermore, the control winding of the stator winding 322 generates a magnetic control field with which the position of the rotor 31 can be regulated with respect to the stator 32.

The fluid F is sucked through the inlet 30 of the rotary pump 3 and conveyed through the outlet 41 into the pressure line 5 by the rotation of the rotor 31 and the fluid is available there under the forwarding pressure, for example 0.5 bar up to 1 bar. A small part of the fluid F flows through the bore 314 (as the double arrow at the lower end of the bore 314 in accordance with the illustration indicates in FIGS. 2 and 3) and thus ensures that the rotor 31 is relieved with respect to the axial thrust.

Depending on which of the valves 8 is or are opened, the fluid F reaches the individual tools T through the lines 71 from the pressure line 5. The rest of the fluid F, which is not dispensed to the tools T, enters back into the supply tank 2 via the pressure reducing valve 9 and the inlet 6, whereby a recirculation of the fluid F and thus a mixing in the supply tank 2 is realized.

In addition to this pump function, the rotary pump 3 also directly produces a mixing of the fluid F in the supply tank 2, because the vanes 311 projecting into the supply tank 2 act as stirring tools and mix the fluid 2 in the supply tank 2.

Since in this embodiment of the dispensing apparatus 1 in accordance with the invention (FIG. 2), unlike known dispensing apparatuses (FIG. 1), the forwarding pressure is generated by the rotary pump 3 and not by applying pressure to the fluid by a gas G, a much simpler design results. Moreover, the supply tank 2 does not have to be designed impermeable to gas, which makes a simpler refilling possible. In addition, there is no risk that the gas G service as a pressure medium will contaminate the fluid F in the supply tank 2.

The control unit 12 preferably adjusts the forwarding pressure of the rotary pump 3 via the speed of the rotor 31, which will be explained in the following.

In European patent application No. 01810790.4 of the same applicant, it is proposed to operate a rotary pump at an efficiency which is much smaller than the maximum efficiency of the rotary pump, for example at the most 20 percent of the maximum efficiency.

The term efficiency is to be understood as the hydraulic efficiency of the rotary pump, that is, the ratio of hydraulic performance (conveying performance) of the pump and mechanical performance for the drive of the rotor (without any friction losses which may be present in bearings or similar).

In European patent application No. 01810790.4, whose contents are herewith incorporated into the present application, it is explained that there is a clear relationship for such operating ranges of the rotary pump 3, in which the efficiency is clearly below the maximum efficiency, between the speed of the pump rotor and the forwarding pressure (delivery head) and also between the speed and the volume flow (flow). In these operating ranges, the forwarding pressure is approximately proportional to the square of the speed of the rotor. This opens up the possibility of setting or regulating the forwarding pressure directly and without an additional pressure measurement via the speed of the rotor 31.

The exact mathematical relationship between the forwarding pressure and the speed, which naturally also depends on the properties of the fluid F, does not need to be known. It is only important that this relationship is biunique for such operating ranges in which the rotary pump 3 operates at a very low efficiency. For example, calibration measurements are carried out in advance in order to determine the pressure/speed curve. This curve can then be stored in a memory of the control unit 12 as a mathematical function, e.g. a polynomial approximation, or as an electronic look-up table. During the operation of the rotary pump 3, the associated speed for the desired forwarding pressure is then looked up in the look-up table. The desired forwarding pressure can then be realized by setting the corresponding speed.

The biunique relationship between the speed and the forwarding pressure or between the speed and the volume flow naturally also depends on the fluid F to be conveyed, in particular also on its viscosity. In the already cited European application No. 01810790.4, it is therefore proposed to use the biunique relationship which exists in operating ranges with very low degrees of efficiency to determine the viscosity or the dynamic viscosity of the fluid. Reference is made in this respect to the explanations in this European patent application. In basically the same manner, the dispensing apparatus 1 in accordance with the invention can also be used in order to determine or monitor the properties of the fluid F such as its viscosity or also its density or other parameters which can be derived therefrom. The possibility is thus opened up of additionally carrying out a quality control of the fluid online or inline with the dispenser apparatus 1 in accordance with the invention.

The determination of the viscosity of the fluid F takes place by means of the motor current with which the rotation of the rotor 31 is driven. The motor current is directly a measure of the torque with which the rotor 31 is driven. In particular in the case of the preferred aspect of the rotary pump as a bearing-free motor, no mechanical bearing friction is present due to the magnetic mounting of the rotor so that the torque with which the rotor is driven coincides in very good approximation with the torque transmitted to the fluid.

Due to the very low efficiency with which the rotary pump 1 is operated in the operating state described here, practically the whole torque and thus the mechanical performance which the impeller or the rotor 31 transmits to the fluid is converted into liquid friction losses. The torque of the rotor 1 is thus directly a measure for the viscosity, more precisely for the dynamic viscosity of the fluid, because the mechanical performance of the rotor 31 is almost completely converted into friction losses of the fluid.

As already mentioned, the torque which the rotor transmits onto the liquid substantially, that is, except for mechanical friction losses, corresponds to the drive torque with which the rotor is driven. This applies in particular to magnetically mounted rotors. The drive torque is again given by the motor current which drives the rotor. The motor current is understood to be the torque-forming portion of the current, also known as the armature current, in the electrical drive. The armature current is in particular a very good measure for the torque with which the rotor is driven in field-oriented three-phase motors.

There is thus a direct connection between the motor current with which the pump is driven and the viscosity of the fluid in the operating range in which the rotary pump 3 only operates at a fraction of its maximum efficiency. The dynamic viscosity of the fluid to be conveyed can thus be determined in a simple manner and online by a measurement of the motor current.

In basically the same manner, other properties of the fluid, for example its density or the mixing ratio of two components of the fluid F, can also be determined when the rotary pump 3 is operated in such operating states in which it has a very low efficiency.

Particularly with regard to the quality monitoring or the determination of parameters of the fluid F, it can be advantageous for a temperature sensor 315 (see FIG. 3) to be provided, for example at the outside of the rotor housing 312, with which the temperature of the fluid F can be detected.

In such operating regions in which the rotary pump 3 only operates at a fraction of its maximum efficiency, the forwarding pressure at which the fluid F is made available in the pressure line 5 can therefore be set and regulated directly by the control unit 12 via the speed of the rotor 31. An extremely fast electrical or electronic adjustment of the forwarding pressure can thus be realized. The forwarding pressure can be regulated, for example, in time intervals of less than 100 milliseconds.

FIG. 4 illustrates a further embodiment which is in particular suitable for applications in which the viscosity of the fluid or other of its properties are to be determined. In the following, only the differences with respect to the embodiment in accordance with FIG. 2 will be dealt with. The reference numerals have the meaning already introduced.

In this embodiment, means are provided to regulate the level of the dispensing tank 2. These means include a tank 13, a connection line 14, which connects the tank 13 to the dispensing tank 2, and a constant gas volume 15, which is provided in the dispensing tank 2. This gas volume 15 can naturally also be zero. The purpose of these means 13, 14, 15 is to keep the level in the dispensing tank 2 constant. The tank 13 has a variable level; it can also be refilled. If fluid F is now taken out of the dispensing tank 2 by means of the rotary pump 3, then fluid flows after it from the tank 13 through the connection line 14. In this way, a constant level FS can be regulated in the dispensing tank 2.

The level regulation in the dispensing tank 2 is in particular advantageous when the viscosity or other properties of the fluid F are to be determined or monitored with the dispensing apparatus 1 of the invention. It is namely ensured by the constant level FS in the supply tank 2 that always just as much fluid F or liquid (that is the same amount) is subjected to stirring power in the dispensing tank 2. This stirring power is therefore particularly a good measure for the specific liquid friction and thus represents an exact measure for the viscosity. A much more precise determination of the viscosity or other properties of the fluid is thus made possible.

It is furthermore possible—as is indicated in FIG. 4—that the supply tank 2 is fed with two—for example, different—components such as liquids and/or gases, namely with a first component which flows from the tank 13 through the connection line 14 into the supply tank, and with a second component which flows through a further line 16 from a further tank (not shown).

The supply tank 2 then serves as a mixing tank in which the two components are mixed to form the fluid. In basically the same manner as was explained for the viscosity, the mixing ratio of the two components can be monitored or checked by the dispensing apparatus 1 of the invention.

Furthermore, FIG. 4 shows another variant for the aspect and the arrangement of the rotor 31. Here, the rotor 31 is designed and arranged such that its vanes 311 do not extend into the dispensing tank 2. The opening B or the diameter B of the inlet 30 here also amounts to more than 50 percent of the diameter of the rotor 31 in order to achieve a good mixing.

FIG. 3 shows another variant for the supply tank 2. The reference symbols have the same meaning which was explained with respect to FIG. 2. In this variant, the rotor housing 312 at the base of the supply tank 2 and the vanes 311 have been somewhat modified. Moreover, static guide elements 21 are provided in the supply tank 2. These have the effect of guiding the fluid currents generated by the rotary pump 3 further upwards (in accordance with the illustration with respect to FIG. 3), as is indicated by the arrows with the reference symbol Z. In particular with larger supply tanks 2, it can be ensured by such a measure that the constant mixing of the fluid F covers the total supply tank 2 and does not remain limited locally to the vicinity of the rotor 3.

The dispensing apparatus 1 in accordance with the invention is advantageous in particular for such fluids F which incline to clumping, phase separation, precipitation or deposition of particles, for example for suspensions, especially for slurry solutions. The fluid F located in the supply tank 2 remains in movement due to the recirculation and the stirring effect produced directly by the rotor 31 so that a constant mixing takes place.

The dispensing apparatus 1 in accordance with the invention is naturally not limited to the application described here, namely to the conveying of a slurry suspension or to CMP processes. It is also generally suitable, among other things, for the conveying of suspensions, emulsions, paints, foodstuffs (e.g. juices or concentrates).

A particular advantage is the combination of pump function and stirring function, with the forwarding pressure being able to be adjusted and regulated in a very simple manner and extremely rapidly in an electronic manner. 

1-15. (canceled) 16: A dispensing apparatus for a fluid comprising a supply tank for the fluid having an outlet, a pressure line between the outlet and an inlet into the tank, a rotor disposed in the outlet in a space fluidly connected with the pressure line for causing a flow of pressurized fluid through the pressure line towards the inlet of the tank and for simultaneously mixing fluid in the tank, and an electric drive for rotating the rotor comprising a stator arranged about a portion of the rotor which magnetically journals the rotor in a contact-free manner relative to the stator and causes rotation of the rotor when electric current is applied to the stator. 17: A method of dispensing a fluid from a tank having an inlet and an outlet comprising placing a rotor in the outlet, surrounding a portion of the rotor with a stator, magnetically journaling the rotor in the stator in a contact-free manner, electrically energizing the stator to thereby rotate the rotor while the rotor is journaled in a contact-free manner, and using the rotor to induce a flow of pressurized fluid from the outlet to the inlet of the tank. 18: A method of dispensing a fluid from a tank having an inlet and an outlet comprising providing an electric motor having a rotor and a stator surrounding the rotor, placing the rotor in the outlet, magnetically journaling the rotor in the stator, energizing the motor to rotate the rotor while it is magnetically journaled in the stator, and with the rotating rotor inducing a pressurized fluid flow from the outlet to the inlet of the tank. 19: A method according to claim 18 including mixing the fluid in the tank with the rotor. 20: A method for regulating the pressure in a fluid conduit between an outlet and an inlet of a tank holding a fluid comprising providing an electrically driven pump including a rotor and a stator surrounding a portion of the rotor, magnetically positioning and journaling the rotor in the stator in a contact-free manner, placing the rotor in the outlet in a space fluidly connected to the conduit, energizing the stator to rotate the rotor at a sufficiently low speed at which a pressure in the fluid flowing into the conduit is approximately proportional to the square of the speed of the rotor, establishing a relationship between the speed of the rotor and the pressure of the fluid in the pressure conduit over a range of rotor speeds at which the fluid pressure on a discharge side of the rotor is approximately proportional to the square of the rotor speed, operating the pump to transfer fluid from the outlet to the inlet, and setting a desired pressure for the fluid flowing in the conduit by rotating the rotor at a speed at which the fluid pressure is at the desired pressure. 21: A method for determining a physical property of a fluid in a tank having an outlet and an inlet and a conduit connecting the outlet and the inlet comprising providing an electrically driven pump having a rotor and a stator, the rotor being magnetically supported and journaled relative to the stator in a contact-free manner, placing the rotor in the outlet so that the rotor induces a pressurized fluid flow through the conduit from the outlet to the inlet when the rotor rotates, electrically energizing the motor to rotate the rotor at a low speed at which a pressure of the fluid in the conduit is approximately proportional to the square of the speed of the rotor, measuring a torque generated by the motor to rotate the rotor, and from the measured torque determining the physical property of the fluid. 22: A method according to claim 21 wherein measuring the torque comprises measuring an electric current consumed by the motor. 23: A method according to claim 21 wherein the physical property comprises a viscosity of the fluid. 24: A method according to claim 21 wherein the physical property comprises a density of the fluid. 25: A method according to claim 21 wherein the fluid comprises a plurality of fluid components, and wherein the physical property is a mixing ratio of the plurality of fluids in the tank. 26: A method according to claim 21 including maintaining a constant fluid level in the tank during measuring the torque. 27: A method according to claim 26 including mixing the fluid in the tank with the rotor to maintain a substantially homogeneous fluid mixture in the tank. 